Motivation:

After enabling the thread sanitizer during testing, a data race with the `activeConnectionCount` in `RedisMetrics` was caught due to changing a primitive `Int` across threads.

Modifications:

Add a specialized class `ActiveConnectionGauge` to wrap the semantics of the `activeConnectionCount` with an `Atomic<Int>` property.

Result:

No data races should occur with the `RedisMetrics` subsystem.